coastal upwelling

Phytoplankton diversity and nitrogen use in an upwelling system

figure 1:A) Drawdown of nitrate (NO3-; blue line) and accumulation of phytoplankton biomass (green line) over the course of a week-long mesocosm experiment in Monterey Bay designed to simulate upwelling conditions. B) Specific rates of nitrate uptake (VNO3) by the large (predominantly diatoms; red line) and small (grey line) phytoplankton size fractions, and the total phytoplankton community (green line) during the mesocosm experiments. Figure adapted from Fawcett and Ward, 2011.

The ocean’s most productive regions are associated with eastern boundary currents where the upwelling of cold, nutrient-rich waters from depth stimulates phytoplankton blooms. The phytoplankton community in these regions tends to be dominated by diatoms, although the exact mechanism for diatom success has never been explicitly demonstrated. By simulating upwelling events in large mesocosms and monitoring nutrient uptake, biomass concentrations, and phytoplankton community composition over the course of the subsequent bloom, we investigated the response of diatoms (along with the entire phytoplankton community) to upwelling conditions (figure 1). We found that the biomass was initially dominated by the smallest phytoplankton, but the largest phytoplankton (which light microscopy counts and pigment analyses suggest were mainly diatoms) increased most rapidly to dominate the biomass, and rate of nitrogen assimilation and carbon fixation after less than four days. While all phytoplankton size fractions achieved similar maximum specific nitrate uptake rates (i.e., absolute uptake rates normalized to biomass; VNO3), this occurred most rapidly and was maintained longest by the largest phytoplankton. This strategy appears to be the mechanism by which diatoms exploit upwelling conditions; their initial rapid acceleration and subsequent maintenance of a high VNO3 allows diatoms to dominate algal biomass in upwelling environments as it gives them access to a disproportionate fraction of the available nutrients. Pigment measurements and light microscopy independently documented changing phytoplankton abundance, with diatoms demonstrating a characteristic pattern of succession: initially, the diatom community was dominated by a few ubiquitous oceanic and coastal species that are known to succeed under low nutrient conditions, and diversity was low. As nitrate became available, diversity increased due to an increase in the abundance of upwelling diatom species that were able to efficiently exploit the new environmental conditions. By the time all available nitrate had been consumed, the diatom assemblage was completely dominated by upwelling species, and diversity was once again low.

The mesocosm experiments suggest that the ability of large diatoms to accelerate and maintain elevated nitrate uptake rates explains their dominance over other phytoplankton groups during upwelling. Moreover, the observed delay in biomass accumulation following nitrate supply after the initiation of upwelling events has been hypothesized to result from changes in the diatom community structure or from physiological acclimation. To investigate these hypotheses, we calibrated the “Tracers Of Phytoplankton and Allometric Zooplankton” (TOPAZ) ecosystem model with a coastal upwelling species assemblage (figure 2), and used it as a framework to evaluate the results of the mesocosm experiments. We found that to accurately reproduce the observed patterns and timescales of size-partitioned new production in a non-steady state (i.e., upwelling) environment, variations in functional group-specific traits must be taken into account, through adjustments of group-dependent maximum production rates (PCmax, s-1; figure 3). The model required neither nutrient acclimation nor diatom diversity to simulate the patterns observed in the mesocosms, provided that lower-than-theoretical-maximum production rates were implemented. We conclude that this physiological feature, PCmax, is critical for representing the early, relatively higher specific nitrate uptake rate of large diatoms, and explains the differential success of small and large phytoplankton communities in response to nitrate supply during upwelling.

figure 3: Comparison of the accumulation of particulate nitrogen (PN) biomass and nitrate consumption attributable to the different phytoplankton size fractions as suggested by the model (lines) versus the mesocosm observations (close symbols). In panels A and B, the same theoretical PCmax value is used for all size groups, whereas in panels C and D, each of the phytoplankton size fractions is assigned a different, optimized value of PCmax. Figures from Van Oostende et al., 2015.